MURI Grad Student Teleseminar

Long Pulse Operation of a High Power Microwave Source with a Metamaterial Loaded Waveguide

Xueying Lu

MIT

02/03/2016

Outline

• Review of Stage I experiment

– Jason Hummelt thesis research

• Design and simulation of Stage II structure

– Reversed metamaterial design (MTM-R)

• MTM-R experimental results

– Sample high power pulses

– Operation space exploration

– Tuning of frequency and power

– Improved operation with steering coils

• Conclusions

#2

Experimental setup

Solenoid currentLenscurrent

ElectronBeam

• 3 tunable operation parameters:

– Beam voltage– Lens current– Solenoid current

#3

MTM structure design by J. Hummelt

• Complementary split ring resonators

• Beam travels between two identical MTM plates in a waveguide

Beam

#4

mm

Experimental results

• Low magnetic field (< 450 G), antisymmetric mode, Cherenkov-cyclotron interaction

• Power level up to 5 MW

• High power pulses always observed with short pulse lengths below 300 ns

#5

Reversed (MTM-R) circuit change

• Asymmetry introduced by reversing one of the MTM plates

• Built a complete new structure

• Two modes with negative group velocity

• Two types of interaction:

– Cherenkov , Cherenkov cyclotron .

#6

Eigenmode simulation of field in a unit cell

• Electric field symmetry is changed

– Both modes are hybrid and deflecting in the MTM-R designMTM-R design

Mode 1 Mode 2 Plotted field: one period on the middle cutting plane

Beam direction

#7

CST PIC simulation

• Uneven power in two arms

• CST predicts Cherenkov-cyclotron mode dominates at low magnetic field

Spiraling beamOutput power in two arms at 900 G

#8

Arm 1Arm 2

Sample pulse 1- long microwave pulse

• V = 420 kV, I = 60 A, Bz = 437 Gauss

• Total output power 2.5 MW, efficiency 10%, pulse length 1 μs

• Electron beam interception as expected

– Both Mode 1 and Mode 2 are deflecting modes

• Coherent radiation at a single frequency of 2.35 GHz

#10

NormalizedFFT amplitude

Sample pulse 2- high peak power

• V = 475 kV, I = 79 A, Bz = 450 Gauss

• Peak total power: 8 MW

• Multiple frequencies

#11

Operation space of high power

• MW-level power is found in a band of the solenoid-lens space

• The role of lens field

– Not changing magnetic field in the structure– Affecting beam profile

> MW power level

Low power

Low power

Fixed voltage: 490 kV

#12

Starting voltage measurement

• Starting voltage depends on both the lens field and the solenoid field

• MW level power when operating voltage is above starting voltage

#13

Power variation with solenoid field

• Peak power experimental data taken at:

– A fixed lens field of 787 Gauss– A fixed voltage of 460 kV

00.5

11.5

22.5

33.5

390 410 430 450 470

Pow

er (M

W)

Solenoid field (Gauss)

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Mode 1, Cherenkov,Downshifted 5 MHz

Mode 2, cyclotron,Upshifted 25 MHz

?

Mode 1, cyclotron

Highpower(> MW)

LowPower(<1 kW)

Frequency tuning with solenoid field

• Dots: experiment

– Fixed voltage 460 kV, fixed lens field 725 G

• Lines: calculation from uncoupled dispersion curves

#15

Frequency tuning with voltage: low power

• In the low power regime, excitation of different modes at different solenoid field values is verified.

Solenoid field 648 Gauss

*Theory line upshifted 25 MHz

Solenoid field 1511 Gauss

*Theory line downshifted 5 MHz

Both terms increasewith a higher beam voltage=> Bigger slope

Only one term increases With a higher beam voltage:=> Smaller slope

#16

Frequency tuning with voltage: the new type

• New type of interaction: not tunable with voltage.

– Not the Cherenkov type or the cyclotron type interaction

• Solenoid field 421 G, lens field 725 G

#17

Steering coil installation

• Transverse kick on the beam to help with the cyclotron motion

• Combined effect of longitudinal focusing and transverse deflecting magnetic field

Bs

vs

v0

Steering oil position

Lens

#18

Improved pulse with the steering coil on

• Steering coil current 0.2 A, beam deflecting angle in free space 13 mrad

• Beam voltage 450 kV, full beam current 73 A

• Lens 725 G, solenoid 466 G

• Improved microwave pulse envelope with a higher total power of 2.9 MW

Central: 2.344 GHzBW: 1 MHz

#19

Conclusions

• The MTM-R structure has been built and tested.

• The design mode is a hybrid deflecting mode with a negativegroup velocity with the Cherenkov type and the cyclotrontype interaction.

• A full width 1 μs long microwave pulse was generated with a420 kV, 60 A beam. The total power in the two output armswas 2.5 MW and the efficiency was 10%.

• Frequency tuning measurement shows different types ofinteraction (Cherenkov, cyclotron and a new type) at variedoperation conditions.

• A pair of steering coils was installed and helped increase theoutput power to 2.9 MW with a better flattop.

#20

• Faculty and staff

Sudheer Jawla

Ivan Mastovsky

Guy Rosenzweig

Michael Shapiro

Jacob Stephens

Richard Temkin

Paul Woskov

• Graduate students

Hannah Hoffmann

Julian Picard

Samuel Schaub

Alexander Soane

Haoran Xu

Acknowledgement

#21